Light display devices such as liquid crystal display (LCD) devices generally control the direction of light using optical films with a prism array to enhance brightness. Such light guides are combined to the output side of light source devices such as a cold cathode fluorescent lamp (CCFL) or light emitting diode (LED), and then to the cells of the display devices.
Enhancement of brightness can be achieved by an optical film with such a prism array, preferably by reflection and refraction processes including light regeneration. When used together with a light source device, a film having a prism-structured surface collects light, which is emitted at high angles from the output side of the light source device, in a vertical direction and directs light to viewers.
Examples of representative Brightness Enhancement Films (BEF) are those that are available from 3M Company located in Saint Paul, Minn., U.S., which typically comprise a prism array larger than a wavelength of light on one surface thereof. Such a structure is helpful in increasing the amount of light directed toward the viewers. These films are used in combination with light source devices, and thus, typically sacrifice the one- or second-dimensional reserved lighting to increase the axial lighting. By doing so, these films decrease the consumption of electricity and help achieve the desired amount of axial lighting.
Further, there already exist privacy films that are also available from 3M Company, etc., which function to allow only the front angle to be viewed and block both sides from being spied. These films are made by inserting dyes in the longitudinal direction of prisms to prevent both side angle views from being spied.
The optical films with a conventional prism array such as BEFs and privacy films guarantee brightness for the front angle view and have a symmetric property for both sides, but not for the front. Thus, it may be inappropriate to use the existing optical films in light source devices having different brightness depending on the direction (hereinafter, “directional light source devices”), displays having different brightness depending on the angle of view, three-dimensional displays, or holograms showing different images depending on the angle of view, etc.
For users to see three-dimensional images that are recently popular, special glasses are generally used to allow both eyes to see different images from each other. Recently, studies on glass-free three-dimensional displays, which require no special glasses, have been actively conducted. One representative technology is a lenticular or parallax barrier autostereoscopic technology. However, such technologies have shortcomings since the film manufacture is complicated and the left and right eye images are periodically changed depending on the direction, thereby causing nausea to the users depending on the angle of view. A recently developed method, which combines prisms and lenses, also has problems with commercialization due to complicated processes.